paths.relativize doesn't support a path not fully under start

The docs for `paths.relativize` compares itself to `os.path.relpath`, but the current implementation requires `path` be fully under `start`.

In theory it could also work for things that have a common prefix:
* `paths.relativize("foo/bar/baz", "foo/mumble")` could return `"../bar/baz"`
* `paths.relativize("foo/bar", "foo/mumble/baz")` could return `"../../bar"`
* `paths.relativize("foo/bar/baz", "mumble/grumble")` could return `"../../foo/bar/baz"`

Right now these cases `fail`, so adding the support shouldn't break anyone. If callers want something fully nested, they could check the result to ensure it doesn't start with `..`.

Start at the paths.relativize entry point and inspect how the current implementation handles paths that are not fully under start. Use the three examples in the issue as the behavioral checks; done means each returns the shown relative path instead of failing, without changing fully nested behavior.

Written by the indexing model from the issue text.